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Under a Microscope

In Review: The Leafcutter Ants: Civilization by Instinct

There are few spectacles of nature as fascinating as a long line of leafcutter arts marching along a well-beaten trail, each of them carrying a piece of a leaf many times their own size, making a tight and well ordered column of flashing green. As you would guess, there’s more happening in that scene that mere foraging. What you’re seeing is a highly choreographed common endeavor, one of the more complex biological organizations on the planet.

The subtitle of Bert Hölldobler and EO Wilson’s latest book on ants says it all: “civilization by instinct.” These two well known authorities on ants tell the story of the leafcutters in exacting detail, and in the process reveal how the line of troopers in the woods is nothing short of epic.

Hölldobler and Wilson received a Pulitzer Prize for their extensive 1990 book, The Ants. Their newest book, The Leafcutter Ants, is essentially an expanded chapter from that larger work. Thoroughly researched and well documented, the newer book is suitable for lay reading and experts alike. But the references tend to get in the way of the reading, and the wealth of Latin names used to describe the behavior of different species of leafcutter ants – found in both the New World and the Old World tropics and subtropics – can become a bit difficult to remember. Fortunately, the written descriptions of ant behavior and organization are bolstered by excellent black and white photographs and diagrams, as well as a handy glossary of terms. Overall, this is a book that is as captivating as the ant farm you might have spent hours watching as a kid.

The leaves the ants carrying are not, in turns out, their food. Rather, those bits of leaf are destined to be laid down and “farmed” as a growing bed for fungus, the ants’ principal food source. The leaves are transported to elaborate nests that stretch for many feet underground, with chamber upon chamber dedicated to this form of insect agriculture. The complexity of this process is mind-boggling, but it occurs without the intelligence we ascribe to such activity in “higher” animals and humans.

The book works through all aspects of the biology and behavior of leafcutter ants. Some species have more advanced organization than others. For example, one species not only has the worker ants that cut and move the leaves into the nest, but includes smaller “fighter” ants that ride piggyback on the worker to fend off attacks from parasitic flies. At the center of the nest is the bloated queen ant, which lays the eggs that provide the colony with workers.

Then there are the predatory ants that try to steal the fungus from leafcutter ant colonies. These specialized pirates will invade a nest, take it over, eat all the leftover ant pupae and fungus, and then move on to invade another ant nest. Others are more sneaky, living as parasites within the leafcutter ant colony itself.

How does it all work? The ants mostly communicate chemically; the trails from leaf sources to nest, for example, are marked by chemicals. They also use sound: the ants rub body parts together (“stridulation” is the technical term) to produce vibrations that other ants pick up via the ground, rather than through the air. The ants even secrete antibiotics to control invading fungi that threaten their food source.

Toward the end of The Leafcutter Ants, the authors conclude: “There can be little doubt that the gigantic colonies of the Atta leafcutters, with their interlocking symbiont communities and extreme complexity and mechanisms of cohesiveness, deserve special attention as the greatest superorganisms on Earth discovered to the present time.”

The “discovered at the present time” qualifier here is important. Wilson has pointed out that our knowledge of the insects inhabiting the soil is extremely limited. New species are being discovered all the time, and their interactions, while critical to the functioning of natural ecosystems, are barely understood. There is a lot to be learned from looking down into the dirt.